This invention relates to control of foam in liquid treatment, such as foam occurring in aeration tanks of sewage treatment plants. The invention more particularly relates to apparatus for breaking the foam bubbles and thereby reducing the foam to a liquid.
The foam in treatment plants today is different than it was years ago. The foam of the sixties was caused by a circular molecular detergent chain which was not readily biodegradable and which would continue to build up on the aeration tanks until in some cases, the foam would break loose in a moderate breeze and float through the air. This light consistency was caused by the chemical base of the detergents which the manufacturers were able to correct in 1963, which occurred after a short but very concentrated industry-wide study to find a biodegradable substitute detergent.
Most secondary treatment plants entered into an era of relief following the development of the long chain detergents. Those plants, however, which had special industrial or cannery liquid waste discharges were not relieved, and in fact, their problems continued but in a cyclic manner. It was common to ignore these cyclic foam problems by taking action whenever they occurred. This problem was compounded by the large number of secondary plants which were constructed in the seventies, and many of the plant designers were unaware of the subtle differences that the proposed plant influent had from the norm. The resulting periodic foam problems were generally attacked by installing foam spray systems. These systems had reasonable success against the lighter foams but the heavier foams often rose right up over the spraying water and flowed over the walkways.
Foam in some secondary treatment plants was periodically rising up like overworked bread dough until it covered the walkways and ran into the galleries. The work required to clean up the mess became significant. In some instances, the foam was controlled to a degree, with the use of defoamant chemicals. However the cost of chemicals was prohibitive in many installations. Also some plants could not use chemicals because the plants had several strains of filamentous biomass, entering from fish canneries, which were not affected by chemicals.
To avoid the use of expensive chemicals, a vacuum system was tried, consisting of a 6 inch plastic pipe header at the downstream end of each aeration tank to vacuum up the foam and discharge the liquid residual into a sewer line. One such system was installed in a full scale test program which revealed that a problem existed in the discharge of the foam into the sewer pipes. Unless the air was removed from the foam, it would not readily flow into the sewer, seriously reducing the rate of removal. Also the power requirement for the vacuum system was substantial.
An article in the November, 1977 issue of the "Water Pollution Control Federation Journal" by K. S. Ng and L. Gutierrez focused on the foam in the paper industry. These authors determined that the important factors in mechanical foam breaking are suction, shear and centrifigal forces. They recommended three blade, 15 to 31 cm diameter, vaned turbines operating in the range of 600 to 1400 RPM. These turbines reduced the heavy foam from paper pulp wastes using a tip speed of about 1400 Cm/sec. While satisfactory for their intended purpose the turbines are relatively expensive both in initial cost and in operating cost.
Accordingly, it is an object of the present invention to provide a new and improved foam breaker which will satisfactorily reduce foam in aeration tanks while being simple in design and operation and being inexpensive to build and install and operate.
It is a particular object of the invention to provide a new and improved foam breaking system based on mechanically shearing or bursting of foam bubbles. A further object is to provide a foam breaker which can be utilized to break the bubbles in the foam in the aeration tank itself, allowing liquified foam to flow from the aeration tank into subsequent treating tanks, such as a sedimentation tank while the liquified foam is still in suspension. An additional object of the invention is to provide a foam breaker which is self-feeding so that additional energy for feeding is not required.
It is a particular object of the invention to provide a foam breaker which does not utilize vacuuming or chemicals or turbines.
An additional object of the invention is to provide such a foam breaker which can be floated directly on the surface of the tank containing the foam. An additional object is to provide such a foam breaker which can be mounted on the wall or railing around the tank and which can be readily moved as desired.
These and other objects, advantages, features and results will more fully appear in the course of the following description.